Explanation

Plasmids, cosmids or bacteriophages can be used as vector in genetic engineering. Plasmids are most widely used circular, extrachromosomal DNA segments seen in the bacterial cells. They carry a foreign gene or desired gene to the host. The size of plasmids ranges from $1 \times 10^{6}$ to $200 \times 10^{6}$ daltons. Among other vector plasmids are pBR 322, those of pUG series and $T_{i},\ R_{i}$ plasmids of Agrobacterium.